Method of liberation of nitrogen and oxygen from air

ABSTRACT

A method of liberation of nitrogen and oxygen from air on the cycle of average or high pressure and subsequent fractionation, which provides for the division of air to be throttled into two flows, of which the first one is cooled together with air directed into the expander, while the second flow is cooled independently, at least part of said second flow being mixed with said first flow at a temperature close to the critical one, additionally cooled, throttled and directed for fractionation.

United States Patent Basin et al.

[ 5] Mar. 14, 1972 [54] METHOD OF LIBERATION OF NITROGEN AND OXYGEN FROM AIR [72] Inventors: Genrikh Maxovich Basin, ulitsa Gorkogo, 8, kv. 8; Ilya Vladimirovich Gorenshtein,

ulitsa Lenina, 48, kv. 14; Mark Eleazarovicli Lemberg, Prospekt Schevchenko, 8a, kv. 19; Semen Grigorievich Linetsky, Krasnoflotsky pereulok, 9/6; Anatoly Eliseevich Myasoed, ulitsa Gagarina, 8, kv. 23; Yakov Benediktovich Zanis, ulitsa Tolstogo, 13, kv. 3, Odessa, all of USSR.

[22] Filed: Mar. 7, 1969 [21] Appl.No.: 805,183

[52] U.S. Cl ..62/22, 62/28, 62/41,

62/43 [51] Int. Cl ......F25j 3/02, F25j 3/04, F25j 5/00 [58] FieldofSearch ..62/13, 14, 15,38,43

[56] References Cited UNITED STATES PATENTS 2,380,417 7/1945 De Baufre ..6 2/38 AIR COMPRESSOR AND 2,433,508 12/1947 Dennis ..62/38 2,645,103 7/1953 Fausek ..62/38 2,779,174 l/l957 Vesque ..62/l5 3,102,801 9/1963 Fetterman .....62/l3 3,210,950 10/1965 Lady 262/13 3,214,925 11/1965 Becker... .....62/l3 3,246,478 4/1966 Kornemann .....62/1 3 3,327,490 6/ 1967 Grenier ..62/38 Primary Examiner-Norman Yudkoff Assistant Examiner-Arthur F. Purcell AttarneyWaters, Roditi & Schwartz [57] ABSTRACT A method of liberation of nitrogen and oxygen from air on the cycle of average or high pressure and subsequent fractionation, which provides for the division of air to be throttled into two flows, of which the first one is cooled together with air directed into the expander, while the second flow is cooled independently, at least part of said second flow being mixed with said first flow at a temperature close to the critical one, additionally cooled, throttled and directed for fractionation.

2 Claims, 2 Drawing Figures ARGON FRAC T/ON -/MPUR/TY REMOVAL l EXPANSION ENG/NE METHOD OF LIBERATION OF NITROGEN AND OXYGEN FROM AIR The present invention relates to methods of liberation of nitrogen and oxygen from air and, more particularly, to methods of liberation of nitrogen and oxygen from air by liquefaction of air on the cycle of average or high pressure and subsequent fractionation.

Prior art methods of liberating nitrogen and oxygen from air with the separation of an argon fraction provide for cooling of the air by the fractionation products and the subsequent expansion in an expander and by way of throttling it, prior to being delivered for fractionation. When so doing, part of air to be throttled is cooled by nitrogen and oxygen, together with that part of the air which is directed to the expander, in a first heat exchanger, while the other part of the stream of air to be throttled is cooled independently by the argon fraction in a second heat exchanger. The temperature difference between said two parts of the air being throttled is to 50 C. (when leaving the heat exchangers). Before entering the throttle valve, both parts of the air to be throttled are mixed.

The transfer of heat from the initial air stream to the argon fraction is accompanied by considerable energy losses because of the irreversibility of heat exchange caused by the great temperature differences in the cool zone of the second heat exchanger. Said differences (2050 C.) occur due to a sharp increase in the heat capacity of air at pressures and temperatures close to the critical pressure and temperature, and due to an insignificant change in the heat capacity of the argon fraction passing through said heat exchanger at a relatively low pressure.

Energy losses also occur due to the irreversibility of the process of mixing the streams of air to be throttled, caused by the difference in the temperatures of said streams when leaving both heat exchangers prior to mixing.

ln order to improve the thermodynamic characteristics of the heat exchange system, it is necessary to reduce the temperature difference in the cool zone and to carry out the process of heat exchange with the optimum temperature difference throughout the entire height of said second heat exchanger, thereby bringing the temperatures of the streams of air to be throttled at their outflow from both heat exchangers closer to one another.

At the same time, it proves impossible to attain the abovedescribed variation of the temperatures in the heat exchange system by reducing the amount of air to be throttled and exchanging heat with the argon fraction, and correspondingly increasing the amount of air to be throttled and exchanging heat with nitrogen and oxygen, since there occurs an increase of the temperature difference in the hot zone of said second heat exchanger in which the air to be throttled is cooled by the argon fraction. This brings about a disturbance of the heat balance and an increase in the amount of refrigeration losses in the air separation cycle, as well as an increase in the value of the required working pressure and in the amount of energy losses.

An object of the present invention is to reduce energy losses in a method of liberation of nitrogen and oxygen from air.

In accordance with this and other objects, the present method of liberation of nitrogen and oxygen from air by liquefaction of air on the cycle of average or high pressure and subsequent fractionation provides for cooling the air with the fractionation products, expanding the cooled air in an expander and throttling said air, which is then divided into two streams, the first of said streams being cooled together with air directed into the expander, and the second stream being cooled independently.

According to the invention, at least part of said second stream at a temperature close to the critical temperature is mixed with said first stream, additionally cooled, throttled and fractionated.

It is expedient to direct for mixing and additional cooling with said first stream such a part of said second stream, after adding which the temperature at the end of said additional cooling and the temperature at the end of the process of cooling the remaining part of said second stream are equal.

The present invention makes for improvement in the then modynamic efficiency of the process of liberation of nitrogen and oxygen from air and, consequently, for a reduction in the working pressure. At the same time, the consumption of electric energy is lowered. The transfer of air from one heat exchanger into another makes the operation of the installation smoother since it diminishes the effect of an inaccurate distribution of air between the heat exchangers upon the operation of the heat exchange system and, consequently, facilitates the operation.

The specific features and advantages of the present invention will appear more completely from the following description of two typical embodiments of installations designed for carrying out the method of liberation of nitrogen and oxygen from air, according to the invention, particularly when read in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of the installation designed for carrying out the method of liberation of nitrogen and oxygen from air, according to the invention; and

FIG. 2 is a schematic diagram of another embodiment of the installation designed for carrying out the method of liberation of nitrogen and oxygen from air, according to the invention.

Communications are designated in the drawings as follows:

Referring to H6. 1, air compressed in a compressor (not shown in the drawing) to 3-5 MN lsq. m. and cleaned of moisture, carbon dioxide and other high-boiling admixtures, at a temperature of 5-35 C. is directed for cooling via pipeline ll into heat exchangers 2 and 3.

Prior to entering the heat exchangers, the air is divided into two parts, a greater and a smaller one. The greater part of air (-80 percent of the total amount) is directed via pipeline 4 into heat exchanger 2, where it is cooled by counterflows of nitrogen and compressed oxygen to l00 C. Then, this part of the air is subdivided into two parts: one part (the first stream to be throttled) to be later throttled, and the other part directed for expansion into an expander 5. The latter of these two parts, constituting 50 to 60 percent of the total amount of air, is directed via pipeline 6 into the expander 5 where it is expanded, performing useful work, to the pressure in a fractionating column 7 and, via pipeline 8, is directed for fractionation into said fractionating column, while the first part of the air to be throttled is cooled on in the heat exchanger 2.

Said smaller part of the air (the second stream to be throttled), constituting 10 to 20 percent of the total amount of air, is directed via pipeline 9 into the heat exchanger 3 where it is cooled by a counterflow of argon fraction to l40l35 C. Then, said second part of the air to be throttled is subdivided into two parts, of which one, constituting 5 to l0 percent of the total amount of air, at a temperature of l40l35 C. (the critical temperature for air) is directed via pipeline 9 for mixingwith the first stream of air to be throttled, which is cooled to the same temperature in the heat exchanger 2, while the second part of the smaller part of the air is cooled in the heat exchanger 3 to l55152 C. After mixing, the parts of the air to be throttled are additionally cooled in the heat exchanger 2 to a temperature ofl55 152 C.

The equality of the temperatures of the flows of air to be throttled at their outflow from the heat exchangers 2 and 3 is attained by adding an appropriate amount of air from that part which is directed from the heat exchanger 3 for mixing into the heat exchanger 2.

The stream of air to be throttled leaving the heat exchanger 2 via pipeline 10 is combined with the stream of air to be throttled leaving the heat exchanger 3 via pipeline 11 and, via pipeline 12, is fed into a throttle valve 13, expanded to the pressure in the fractionating column 7 and directed for fractionation into said fractionating column.

Nitrogen, obtained as a result of the fractionation of air, is fed via pipeline 14, at a pressure of 0.05 MN/dq. m. and temperature of l83l93 C., into heat exchanger 2 where it exchanges heat with air passing in the heat exchanger 2, is thereafter heated to a temperature to l0 C. lower than that of the compressed air entering the heat exchanger 2, and is further directed to the consumer.

Liquid oxygen, obtained as a result of the fractionation of air, is removed from the fractionating column 7 and, via pipeline l5, directed into a liquid oxygen pump 16 with the aid of which, the oxygen, at a temperature of l 83-l 80 C. and under a pressure of up to 20 MN/sq.m., is pumped via pipeline 17 into heat exchanger 2, where by oxygen is heated by the air with the purpose of gasification, and, at a temperature 2 to 3 C. lower than that of the initial compressed air entering the heat exchanger 2, is directed to the consumer.

The amount of nitrogen obtained in accordance with the present method is 65 to 70 percent, while the amount of oxygen obtained to percent, of the total amount of the initial air.

In order to simultaneously obtain high-purity nitrogen and pure oxygen an argon fraction in an amount of 10 to percent of the total amount of air is removed from the fractionating column 7 and directed via pipeline 18, under pressure of up to 0.05 0.05 MN/sq.m., into the heat exchanger 3 where said argon fraction exchanges heat with the air.

In the heat exchanger 3 said argon fraction is heated from a temperature of 185l84 C. to a temperature 4 to 6 C. lower than that of the compressed air entering heat exchanger 3, and then removed to the atmosphere or used for recovering the adsorbent of a cleaning unit (not shown in the drawing) in which initial air is cleaned of moisture and other high boiling admixtures.

In another embodiment of the installation for carrying out the method of liberation of nitrogen and oxygen from air, there is directed into the heat exchanger 2 (FIG. 2) for mixing with the first stream of air to be throttled via pipeline 9, the entire second stream of air to be throttled, which constitutes 10 to 20 percent of the total amount of air. Mixing is effected in the zone of the heat exchanger 2 in which the temperatures of the air streams being mixed are equal l43-l40 C).

Such an embodiment makes the arrangement of the installation simpler however causing a certain increase in thermodynamic losses.

The employment of the present method in an average pressure installation with an output of 600 cu.m/hr of pure nitrogen and and 85 cu.m/hr of pure oxygen facilitates the reduction of the working pressure of air by 0.4-0.6 MN/sq.m and, as a result, the reduction by appr. 3 percent of the electric energy consumption.

We claim:

1. A method for air separation to obtain compressed oxygen and pure nitrogen comprising compressing an incoming stream of air, pruifying the compressed air stream to remove high boiling-point impurities, dividing the resulting compressed purified air stream into a major and a minor stream, cooling the major stream in a first heat exchanger and cooling the minor stream in a second heat exchanger to a temperature near the critical temperature, drawing off a part of the major stream from the middle of said first heat exchanger and expanding the drawn ofi' air, drawing off a part of said minor stream cooled in said second heat exchanger and introducing said drawn off stream into said first heat exchanger where the temperature of said part of said minor stream equals the temperature of the part of the major stream that remains in said first heat exchanger after drawing off the air to be expanded, mixing said two parts, cooling the resulting stream in said first heat exchanger, cooling the part of said minor air stream that remains in said second heat exchanger, the part of said minor stream to be introduced into said first heat exchanger being selected soas to ensure the equality of the temperatures of the streams existing from said first and said second heat exchangers, combining the streams of compressed air cooled in said heat exchanges and expanding said combined stream in a throttling valve, combining the air streams exiting from said throttling valve and said expander, fractionating the combined expanded air stream in a rectifying column to obtain liquid oxygen, high-purity nitrogen and an argon fraction, compressing the liquid oxygen in a pump, heating the oxygen and nitrogen in said first heat exchanger, and heating said argon fraction in said second heat exchanger.

2. A method of air separation to obtain compressed oxygen and pure nitrogen comprising compressing an incoming stream of air, purifying the compressed air stream to remove high boiling-point impurities, dividing the resulting compressed purified air stream into major and minor streams, cooling the major stream in a first heat exchanger, cooling the minor stream in a second heat exchanger, drawing off a part of said major stream from the (middle) of said first heat exchanger and expanding said drawn off part in an expander, introducing said minor stream cooled in said second heat exchanger into the cross-section of said first heat exchanger where the temperature of said minor stream being introduced equals the temperature of the portion of said major stream that remains in said first heat exchanger after drawing off the part to be expanded, mixing said minor stream and said portion of the major stream, cooling the resulting combined stream, expanding said cooled combined stream in a throttling valve, combining the expanded streams exiting from said throttling valve and said expander, fractionating the combined expanded stream in a rectifying column to obtain liquid oxygen, high-purity nitrogen, and an argon fraction, compressing the liquid oxygen in a pump, heating the nitrogen and oxygen product in said first heat exchanger, and heating the argon fraction in said second heat exchanger. 

2. A method of air separation to obtain compressed oxygen and pure nitrogen comprising compressing an incoming stream of air, purifying the compressed air stream to remove high boiling-point impurities, dividing the resulting compressed purified air stream into major and minor streams, cooling the major stream in a first heat exchanger, cooling the minor stream in a second heat exchanger, drawing off a part of said major stream from the (middle) of said first heat exchanger and expanding said drawn off part in an expander, introducing said minor stream cooled in said second heat exchanger into the cross-section of said first heat exchanger where the temperature of said minor stream being introduced equals the temperature of the portion of said major stream that remains in said first heat exchanger after drawing off the part to be expanded, mixing said minor stream and said portion of the major stream, cooling the resulting combined stream, expanding said cooled combined stream in a throttling valve, combining the expanded streams exiting from said throttling valve and Said expander, fractionating the combined expanded stream in a rectifying column to obtain liquid oxygen, high-purity nitrogen, and an argon fraction, compressing the liquid oxygen in a pump, heating the nitrogen and oxygen product in said first heat exchanger, and heating the argon fraction in said second heat exchanger. 